NASA Earth Observatory map by Joshua Stevens
Unique clouds at the edge of space appear to be showing up in spring and summer more often than ever before, according to NASA scientists, who speculate that climate change could be playing a role in cloud formation.
I like the term “noctilucent clouds” for these night-shining clouds glowing with a tint of blue — although NASA researchers formally call them “polar mesospheric clouds.” That’s because they show up at the poles in the mesosphere at about 50 miles up — the outer edge of Earth’s atmosphere. If you are a scientist with a perspective from satellites, you don’t really think about day or night.
Researchers have learned a great deal about these clouds since the 2007 launch of the Aeronomy of Ice in the Mesosphere (AIM) spacecraft, but they still seem distant and mysterious.
Photo: NASA/Dave Hughes
The clouds are actually ice crystals about the size of particles in cigarette smoke, according to an interesting article by NASA’s Tony Phillips, who interviewed cloud-researcher and astronaut Don Pettit in 2003. Because the clouds are so high up, they are seen shortly after the sky turns dark at sunset, a time when sunlight can still bounce off the crystals. Years ago, they were seen only in the far-north latitudes in our part of the world, but more recently they have been seen as far south as Colorado and Utah.
The temperature in the mesosphere is about -125 degrees Celsius, or nearly 200 degrees below zero Fahrenheit. Conditions up there are extremely dry — far dryer than any place on Earth.
Like common clouds in the lower atmosphere, noctilucent clouds need water vapor and a “nucleus” upon which the water can attach. In the lower atmosphere, called the troposphere, ordinary dust and many other particles are common enough as a result of winds. Cirrus clouds can form in the highest layers of the troposphere, about 12 miles up. But until data came back from the AIM project, nobody was sure what was happening at 50 miles up. Now, researchers believe the nuclei are mostly space dust pulled in by Earth’s gravity.
The first reports of noctilucent clouds came in 1885 after the eruption of the volcano Krakatoa. Researchers aren’t sure if volcanic dust made it high enough into the atmosphere to form the clouds, but that potential source disappeared long ago.
Noctilucent clouds are observed in late spring and summer when upwelling winds carry water vapor up into the atmosphere. The increasing frequency of cloud formation may be the result of climate change. It turns out that when greenhouse gases warm the Earth’s surface, the upper atmosphere actually gets colder as heat escapes, helping the tiny crystals to form.
Another factor in climate change could be the increasing amount of methane gas in the atmosphere. A complex series of reactions can oxidize the methane to form water vapor, which can then form ice crystals.
One of the unexpected results of the AIM mission has been unusual “teleconnections” between the north and south poles via the mesosphere. It turns out that a slowing of stratospheric winds over the Arctic affects circulation in the mesosphere, causing a ripple effect around the globe. The southern mesosphere becomes warmer and drier, leading to fewer noctilucent clouds.
These high-level connections were not even suspected when the AIM spacecraft was launched, but they are revealing how weather on one part of the globe may be connected to relatively rapid changes in other far-flung regions. (Check out last year’s video below.) Further studies of the upper atmosphere can be expected to bring more surprises.
